1. Field of the Invention
The present invention relates to a dry etching method employed in such applications as production of semiconductor devices. More particularly, it relates to a dry etching method whereby a polycide film can be anisotropically etched under effective control without using chlorofluorocarbon (CFC) gas.
2. Description of the Related Art
A polycide film, on which a silicon based material layer and a refractory metal silicide layer composed of tungsten silicide (WSi.sub.x), etc. are fabricated in this order, shows a nearly one-digit decrease in resistance over a single-layer polysilicon film of the same sectional area and has been used widely as LSI gate electrode material. Commonly, the silicon based material layer is an impurity-doped polysilicon layer.
However, the polycide film has created new difficulties in a dry etching process wherein both the two dissimilar material layers thereof must be anisotropically etched. Namely, patterns formed of the polycide film are likely to have such etching defects as undercut and narrowing partly because the lower polysilicon layer is etched at a higher rate than the upper refractory metal silicide layer due to the difference in the vapor pressure of a halogen compound resulting from an etching reaction and partly because a reaction layer is formed on the interface between the two layers. Such etching defects are unallowable in the production of submicron devices because the defects form an off-set region, which will not be doped with impurities when ions are implanted to form source and drain regions, or the defects will reduce the dimensional accuracy of a sidewall for forming an LDD structure. To overcome these difficulties, therefore, extensive researches have been conducted to develop a method of anisotropically etching of polycide films.
Conventionally, CFC gas, typically CFC 113 (C.sub.2 Cl.sub.3 F.sub.3), has been widely used as an etching gas for polycide films. CFC gas, which has fluorine and chlorine atoms in each molecule thereof, will emit such radicals as F* and Cl* and such ions as CF.sub.x.sup.+, CCl.sub.x.sup.+, to Cl.sub.x.sup.+ to assist an etching reaction and deposit a carbonaceous polymer on the sidewalls of patterns to provide sidewall protection effects, and thus to achieve high etchrate and anisotropy.
However, CFC gas is commonly known to contribute to the destruction of the earth's ozone layer. Therefore, there is a pressing need to find some appropriate alternative substance for the CFC gas, and the efficient applications of this substance in dry etching.
One of the promising CFC-free etching methods is low-temperature etching. This etching method is designed to keep a target substrate (wafer) at temperatures below 0.degree. C. and thereby keep a longitudinal etchrate at a practical level with the assistance of ions while freezing or inhibiting a radical reaction on the sidewalls of the patterns and preventing such etching defects as the undercut. A typical Example of this method is publicized, for instance, in the Extended Abstract of the 35th Spring Meeting of the Japan Society of Applied Physics and Related Societies, 1988, p. 495, 28a-G-2. In this instance, a silicon trench and an n.sup.+ type polysilicon layer are etched by using SF.sub.6 gas with a wafer cooled to -130.degree. C.
However, this etching method attempts to achieve high anisotropy only by freezing or inhibiting a radical reaction on the sidewalls of the patterns and involves cooling the wafer to such a degree that the method requires liquid nitrogen. Hence, hardware related problems, like decreased reliability of a vacuum sealant, as well as considerable reduction in economy and throughput, makes it difficult to put this method to practical use in the near future.
A more practical etching method might be to combine radical reaction inhibition with sidewall protection at low temperature to perform etching in a temperature zone close to room temperature.
The present inventors have proposed a great number of sidewall protection methods using sulfur (S) deposition. Sulfur will be deposited from the etching gas when the gas is composed mainly of sulfur halides with a relatively high S/X ratio (i.e. the ratio of the number of sulfur atoms to that of halogen (X) atoms in one molecule).
For instance, the present inventors have proposed S.sub.2 F.sub.2 as one of these sulfur halides in the proceeding on tile 4th MicroProcess Conference (1991), p. 32. Unlike the more well-known sulfur fluoride SF.sub.6, S.sub.2 F.sub.2 will form sulfur in a plasma when dissociated by electric discharges. When a substrate is cooled to temperatures below room temperature, the sulfur thus formed will deposit on the surface thereof, providing sidewall protection effects . When the substrate is heated after completion of the etching, the sulfur deposits will sublime immediately, avoiding the danger of inducing particle pollution.
Further, the present inventors have also proposed polysilicon gate electrode forming process wherein a silicon based compound layer is etched by an etching gas containing such sulfur halides as S.sub.2 F.sub.2, S.sub.2 Cl.sub.2, and S.sub.2 Br.sub.2 and such halogen consuming compounds as H.sub.2, H.sub.2 S, and silane based compounds. F* will etch silicon based compounds spontaneously even in the absence of ions. This can be accounted for by the fact that Si--Si bond has a small bond energy of 54 kcal/mole compared with 132 kcal/mole for an Si--F bond and that fluorine, having a small atomic diameter, easily enters the crystal lattice of the single-crystal silicon. To reduce the action of F*, therefore, fluorine radical consuming compounds capable of emitting H*, Si*, and other F*-capturing radicals are added to the above-mentioned sulfur halides in the etching gas to increase the apparent S/X ratio of the etching system and promote sulfur deposition. In this etching process, the present inventors have succeeded in forming a polysilicon gate electrode into a highly anisotropic shape on a wafer cooled to -70.degree. C.
Thus, the present inventors have discovered that this etching method using sulfur halides like S.sub.2 F.sub.2 can form a polysilicon gate electrode into an anisotropic shape in much more practical temperature zones than conventional low-temperature etching. Therefore, it can be expected to apply those sulfur halides to a polycide gate electrode forming process. The most popular polycide film is a tungsten polycide film (hereinafter referred to as "W polycide film") whose upper refractory metal silicide layer is formed of tungsten silicide (WSi.sub.x). On the other hand, the most practical sulfur halide is sulfur fluoride, considering the vapor pressure of a refractory metal halide resulting from an etching reaction.
The present inventors have also discovered, however, that this process is likely to cause critical dimensional losses between a resist mask and a polycide gate electrode. This problem is described by referring to FIGS. 1a to 1d.
Referring first to FIG. 1a, a silicon substrate 1 is coated on the surface thereof with a gate oxide film 2. The gate oxide film 2 is provided on the surface thereof with a polysilicon layer 3 doped with impurities and a WSi.sub.x layer 4, which are fabricated in this order to constitute a W polycide film 5. Further, the W polycide film 5 is provided on the surface thereof with a resist mask 6 patterned into a predetermined shape.
Referring next to FIG. 1b, when the W polycide film 5 is etched by using S.sub.2 F.sub.2 with the wafer thus formed cooled to about -70.degree. C., a WSi.sub.x pattern 4a will be tapered immediately upon etching of the WSi.sub.x layer 4.
The WSi.sub.x pattern 4a is tapered as follows. When dissociated by electric discharges, S.sub.2 F.sub.2 will emit a plasma F* which will etch the W polycide film 5 with the assistance of such ions as SF.sub.x.sup.+ and S.sup.+. Consequently, the WSi.sub.x layer 4 will be removed through volatilization in the form of WF.sub.x, SiF.sub.x, etc. However, part of the WF.sub.x will remain on the sidewalls of the WSi.sub.x pattern 4a on extra low- and middle-low-temperature zones with a low vapor pressure. Meanwhile, S.sub.2 F.sub.2 will also emit free sulfur in a plasma when dissociated by electric discharges. When a film composed of a single polysilicon layer is etched, such free sulfur will deposit on the sidewalls of the resulting pattern, providing sidewall protection effects. When the WSi.sub.x layer 4 is etched, however, part of the free sulfur will react with WF.sub.x to form tungsten sulfide (WS.sub.x : x=2 or 3) with a low vapor pressure. Consequently, sidewall protection films 7 each composed of a mixture of WF.sub.x, WS.sub.x, S, etc. will deposit in excessive quantities on the sidewalls of the WSi.sub.x pattern 4a, and will gradually make the width thereof larger than that of the resist mask 6.
Referring then to FIG. 1c, when the polysilicon layer 3 is etched following the WSi.sub.x layer 4 by using S.sub.2 F.sub.2, a polysilicon pattern 3a will be formed in such a manner as to have nearly vertical walls while sidewall protection films 8 each composed mainly of sulfur will be deposit on the sidewalls of the polysilicon pattern 3a. The width of the polysilicon pattern 3a, however, is made larger than that of the resist mask 6, because it directly reflects the lowermost width of the WS.sub.ix pattern 4a.
Referring finally to FIG. 1d, when the resist mask 6 is removed through O.sub.2 plasma ashing, a W polycide gate electrode 5a will be formed in such a manner as to have a deformed sectional shape. At this time, the sidewall protection films 8 each composed mainly of sulfur will be removed from the wafer through ashing in the form of SO.sub.x or through sublimation under the influence of plasma radiation heat. On the other hand, the sidewall protection films 7 each containing WS.sub.x will not be removed but remain on the wafer. WS.sub.x (particularly when x=2) is known to be a very stable, non-water-soluble compound with the resistance to various strong acids. WS.sub.x can be decomposed only by heating it above 1100.degree. C. under a vacuum or to 800.degree. C. in an H.sub.2 atmosphere. However, such heating is almost impossible in the production of semiconductor devices.
Nevertheless , it is preferable to remove the sidewall protection films 7, which, when left on the wafer, may not only hinder formation of an inter-layer insulation film but also cause particle pollution. To this end, it is necessary to set up etching conditions for inhibiting formation of WS.sub.x. The same also holds true of etching a silicide layer containing other refractory metals than W.